The use of wireless communication devices such as telephones, pagers, personal digital assistants, laptop computers, etc., hereinafter referred to collectively as “mobile appliances” or “mobiles,” has become prevalent in today's society. Recently, at the urging of public safety groups, there has been increased interest in technology which can determine the geographic position or “geo-locate” a mobile appliance in certain circumstances.
In the United States, mobile wireless appliance locating equipment is being deployed for the purpose of locating wireless callers who dial 911. Other services in addition to emergency call servicing are contemplated and are referred to as location based services (LBS). Wireless location equipment is typically employed as an overlay to wireless communication networks, thus forming a network overlay geo-location system.
In operation, these network overlay location systems take measurements of radio frequency (RF) transmissions from mobile appliances at base station locations surrounding the mobile appliance and estimate the location of the mobile appliance with respect to the base station locations. Because the geographic location of the base stations is known, the determination of the location of the mobile appliance with respect to the base station permits the geographic location of the mobile appliance to be determined.
In a network-based geo-location system, the mobile appliance to be located is typically identified and radio channel assignments determined by (a) monitoring the control information transmitted on radio channels for telephone calls being placed by the mobile appliance or on a wire line interface to detect calls of interest, i.e., 911, and (b) a location request provided by a non-mobile appliance source, i.e., an enhanced services provider. Once a mobile appliance to be located has been identified and radio channel assignments determined, the location determining system is first tasked to determine the geo-location of the mobile appliance and then directed to report a determined position to the requesting entity or enhanced services provider.
FIG. 1 illustrates a conventional mobile-appliance communication system having base stations 10 a-c for communicating with a mobile appliance 20. Each base station 10 contains signal processing equipment and an antenna for transmitting to and receiving signals from the mobile appliance 20. A Base Station Controller (“BSC”) and/or Mobile Switching Center (“MSC”) 45 is connected to each base station 10 through a wireline connection 41. A mobile appliance location determining sensor 30 a-c, i.e., wireless location sensor (“WLS”), may be positioned at some or all of the base stations 10 to determine the location of the mobile appliance 20 within the signal coverage area of the communication system. A network overlay system is generally composed of two main components. One component resides at the base station that makes measurements on the RF signal emanating from the wireless device, the WLS 30. The second component resides at the mobile switch that tasks the WLS groups to collect data and then uses the data to compute a location estimate. This second component is generally referred to as the Geolocation Control System (“GCS”) 50.
In the normal course of operation, the GCS 50 is tasked by an outside entity, e.g., the Mobile Positioning Center (“MPC”) 40, to generate a location estimate on a particular mobile appliance. The tasking is accompanied by information on the mobile of interest including the serving base station and sector for the call and the RF channel (frequency, time slot, CDMA code, etc.) being used by the wireless communications network to complete the wireless connection. Once the GCS 50 receives this tasking, based on the serving sector, it tasks a set of WLS units 30 to make measurements on the RF emissions of the mobile 20. The WLS units 30 make the measurements and report them to the GCS 50. The GCS 50 then computes a location estimate using a mathematical or data matching algorithm. Alternatively, control signaling on RF or wireline interfaces used to set up calls in the wireless network may be scanned to detect the placement of a call of interest. The signaling that occurs on the RF control channel may be used to determine location. Call setup/channel assignment parameters may also be extracted from the control messaging to determine which traffic channel to use for location related measurements.
Network overlay location systems typically locate a mobile appliance on the traffic channels of a wireless network. The system typically uses sensors employing techniques such as Uplink Time Difference of Arrival (“U-TDOA”). In U-TDOA, the traffic channel assignment information may be provided through a separate process such as providing MOBINFO (IS-41 Mobile Information) parameters passed by the MPC 40 as part of the GPOSREQ (J-STD-036 Geolocation Position Request) message from the MPC 40 to the GCS 50.
Techniques used for geo-location (i.e., AOA, TDOA, etc.) are known and described in the art. One facet of operation that is important in the aforementioned techniques is the process whereby one site, a WLS co-located with the serving base station, is designated as the primary site and sends information bits related to a sample of the received signal to the other sites designated as secondary sites thereby assisting the secondary sites' hearability of a signal of interest to make location related measurements. Various methods have been developed to define and coordinate the associated tasking, detection and reporting functions. One such method is described in U.S. Pat. No. 5,327,144 to Stilp which is hereby incorporated by reference.
In general, network overlay geolocation systems have WLS's deployed at virtually all base station sites to achieve a desired accuracy. A specific problem in prior art methods is that a primary wireless location sensor must be located at the serving base station. Because of cost and other reasons, there is now a desire to put WLS equipment into a subset of the base station sites (sparse network deployment) and still maintain high location accuracy.
In general, U-TDOA location systems use location related measurements from many sites to estimate the location of a mobile. For example, in GSM systems typically 6 or 7 sites participate in the location estimate. One effect of not having a WLS at every site is degradation of location accuracy. For some air interfaces, this may be problematic. For example, where occupied bandwidth is small such as in AMPS and TDMA, the error associated with location estimates where the number of participating sites is reduced becomes unsatisfactory. Air interfaces such as GSM and CDMA do not suffer from this, and mobile appliances operating in these air interfaces possess wider bandwidth and/or are frequency hopped. These features allow surfaces generated from TDOA or AOA to be less corrupted by multi-path (generally, multi-path may be better resolved in the time domain), and therefore, with fewer surfaces, location estimates are generally acceptable.
Another effect of sparse deployment is “no location areas.” “No location areas” are those areas in which a minimum number of WLS cannot detect or measure an attribute of a signal such that the geo-location system cannot estimate a location. Mobile appliances are power controlled by the wireless network. This means that the mobile's transmit power is changed by the network so that that minimum power is transmitted to achieve an acceptable communications link (i.e., the voice quality is acceptable). When a mobile appliance moves close to a base station site, the required transmit power for an acceptable communications link is reduced to a small value. This power control is well known in the art and is desirable because it diminishes co-channel interference and adjacent cell interference where channel reuse is employed, and prolongs the battery life of the mobile appliance. However, if the base station that is serving the mobile appliance does not have a WLS unit (due to the sparse deployment), then there is no WLS to “hear” the mobile at the serving site, and the neighboring site WLS units may not be able to hear the mobile because of the mobile's low transmit power. The result is of this phenomenon is a series of “no location areas” surrounding the base station sites without WLS equipment due to the sparse deployment.
E-OTD is also a known and described location technique whereby timing measurements are made on forward link transmissions by a handset and passed to a central site to calculate a mobile's location using TOA or TDOA methods.
E-OTD operates by making timing measurements on forward link signals with known data sequences. For GSM, the timing measurements are made on the Base Station Control Channels (BCCH). The timing measurements are forwarded via data links from the handsets to a location processor where the timing measurements, along with the locations of the source base stations, allow a location estimate to be made. One key component of the E-OTD approach is the step of “synchronizing” the time base (referencing from a time base or standard) at the transmitting base stations. In general, GSM base stations are not locked to a high accuracy reference; therefore, the timing measurements made by the handsets are not referenced to a common standard. To create the common reference or time base, E-OTD depends on measuring forward link timing from multiple base stations at known locations. The WLS's typically perform this function. In prior art, the WLS's are referred to as Location Measurement Units (LMU). WLS will mean the same as LMU for forward link timing measurement purposes. Additional refinements of the E-OTD method have eliminated the use of the WLS by synthesizing the base station common reference by taking repeated, over-determined timing location estimates on handsets, referred to as an over-determined solution.
While some prior art systems have used E-OTD as an alternative to estimate a location if the U-TDOA method could not achieve a desired accuracy, no prior art has addressed the joint use of E-OTD and U-TDOA raw measurements to obtain a location estimate, when neither method by itself can achieve a location solution or a location solution within a desired accuracy.
In view of these deficiencies, the present subject matter advantageously addresses the determination of a location estimate using both U-TDOA measurements and E-OTD measurements when a location estimate using either U-TDOA or E-OTD is not available or is not sufficiently accurate. The subject matter thus overcomes problems frequently encountered in sparse network overlays or with poor base station locations and provides a statistically independent location solution.
The advantages of the disclosed subject matter will be readily apparent to one skilled in the art to which the disclosure pertains from a perusal or the claims, the appended drawings, and the following detailed description of the preferred embodiments.